For each character, XOR with the corresponding key char (key cycled
via 'i mod kn'):
let xor_cipher key text =
let buf = Buffer.create n in
for i = 0 to n - 1 do
let c = Char.code text.[i] in
let k = Char.code key.[i mod kn] in
Buffer.add_string buf (String.make 1 (Char.chr (c lxor k)))
done;
Buffer.contents buf
XOR is its own inverse, so encrypt + decrypt with the same key yields
the original. Test combines:
- String.length decoded = 6
- decoded = 'Hello!' -> 1
- 6 * 100 + 1 = 601
Tests Char.code + Char.chr round-trip, the iter-127 lxor operator,
Buffer.add_string + String.make 1, and key-cycling via mod.
90 baseline programs total.